Electric or electro-hydraulic steering devices for industrial trucks have been known for a long time. A steering transmitter is activated by means of a steering wheel or a similar steering element which is operated by the driver and which is pivoted in at least one bearing of the industrial truck by means of a shaft section. The steering transmitter in most cases works electrically and as a rule is a potentiometer. The electrical signals are transmitted to a steering control, which converts the steering wheel angle into a corresponding steering angle of the wheel to be steered. In a hydraulic steering device, for example, a pump is activated which operates by a steering valve on a steering cylinder, which effects the direction the wheels turn.
In connection with such a steering device, it is known from EO 0 596 167 to make the transmission ratio variable between the steering wheel and the steering motor, and in fact dependent on the speed of the industrial truck. Preferably, the higher the speed, the “more indirect” is the steering. Thereby stable handling and smooth travel is maintained. From EP 0 722 878 it is further known not to only change the transmission ratio along a specified characteristic curve dependent on the travel speed, but rather store a set of characteristic curves in a steering device in order to establish the transmission ratio, resp. the transmission characteristic curves, with options, e.g. to adjust according to the experience and ability of the driver. From DE 100 08 984 it is known not to change the steering movement of the steerable wheel dependent only on the travel speed, but also on the mass and/or the lifting height of a load picked up with the industrial truck.
Generally the electro-hydraulic or electric steering systems have considerable advantages. The steering forces are reduced and there is a more free space maintained in the knee area of the driver. Additionally it is possible to continuously adjust the steering ratio during operation with little effort.